In rhesus monkeys, the hypothalamic influence on pituitary LH and ovarian estradiol and progesterone secretion has been evaluated. After localized lesions of the anterior hypothalamus or deafferentation to the medial basal hypothalamus of rhesus females, the preovulatory surge of LH and subsequent increase in serum progesterone levels were blocked. An injection of estradiol into these females failed to elicit an LH surge of comparable magnitude and duration to that measured in non-lesioned females. Similar results have been obtained in hamsters. Intracranial infusion of LH-RH into ovariectomized rhesus monkeys causes an increase in peripheral concentrations of LH. Estrogen facilitates this response whereas certain adrenergic-blocking drugs prevent the LH release after LH-RH and estrogen stimulation. The addition of synthetic LH-RH in 10 to the minus 6 power to 10 to the minus 9M power concentrations causes significant LH release into the media of nomolayer cultures from anterior pituitary cells of both monkeys and rats. The role of cyclic AMP and certain steriods in this monolayer culture system is under investigation.